1. Field of the Invention
The present invention relates to a gas sensor which measures a concentration of a predetermined gas component in a measurement gas by a sensor element, and more particularly to a technique of retaining and fixing the sensor element to a contact member.
2. Description of Related Art
Conventionally, in order to know a concentration of a desired gas component in a measurement gas, there have been employed various measuring apparatuses. For example, as an apparatus which measures an NOx concentration in the measurement gas such as a combustion gas or the like, there has been known a gas sensor (an NOx sensor) having a sensor element in which a Pt electrode and a Rh electrode are formed on a layer of a solid electrolyte having an oxygen ion conductivity such as a zirconia (ZrO2) or the like (for example, see Japanese Patent Application Laid-Open No. 2006-284223 (Patent Document 1)).
The sensor element of the gas sensor mentioned above typically has a plurality of electrode terminals on a surface for applying an electric voltage, picking up a detection signal, supplying an electric power to a heater part and the like. Further, the gas sensor is provided with a contact member having a housing in which the sensor element is inserted and retained. The contact member is provided with a plurality of retaining members which are constructed by metal terminals, a plurality of lead wires which are connected to the retaining members and achieve an electric continuity between the sensor element and an external portion, and an insertion port in which the sensor element is inserted. In the gas sensor, the sensor element is inserted in the insertion port and the contact member retains the sensor element in such a manner that the retaining member comes into contact with the electrode terminal, so that the electric continuity between the sensor element and the external portion is obtained. For example, there has been already known a gas sensor in which a retaining member is provided with a plurality of convex parts as a contacting part between the sensor element and the electrode terminal, which are directed to the electrode terminal while a sensor element is inserted, and in which a retention of the sensor element and a reservation of a continuity are carried out by causing an elastic deformation in the convex part with an energization from an outer side (a fastening of a contact member) (for example, see Japanese Patent Application Laid-Open No. 2002-168824 (Patent Document 2)).
In the case of carrying out the retention of the sensor element and the reservation of the electric continuity in accordance with an aspect disclosed in Japanese Patent Application Laid-Open No. 2002-168824 (Patent Document 2), an elastic deformation of the convex part acting as a contact part with the electrode terminal due to the energization from the external portion is caused in such a manner that a contact area with the electrode terminal is increased in comparison with that in a non-contact time. In this case, there is such a problem that, since a surface pressure in the contact part is reduced in accordance with an increase in the contact area and a nonconductor film is formed in the contact part, a contact failure tends to occur. Further, there is such a problem that, since an elastic area (a limit of deforming by an elastic deformation) is small in the convex part in the first place, a plastic deformation tends to be caused. If the plastic deformation is caused, the surface pressure in the contact part becomes smaller in comparison with that in the elastic deforming time. After all, the contact failure tends to be caused.